The present invention concerns an apparatus for the controlled extraction of steam from extraction turbines.
Volume flows are extracted for heating or operational purposes from any stage of extraction turbines. The flow of steam to be extracted can be influenced either by an external or an internal control.
The external control is accomplised by a throttle element that is placed within the extraction line. A flow of steam, in functional relation to the rate of throttling set by the throttling element, always passes through the turbine section located behind the point of extraction. The amount of steam flowing through this turbine section can therefore not be reduced to any quantity desired if this type of control is being employed.
This disadvantage is overcome by an internal control which makes it possible to vary the flow of steam which remains in the turbine behind the point of extraction between zero and 100 percent. In practice, the minimum value of the flow of steam behind the point of extraction will equal the flow of cooling steam which is required for the cooling of the remaining blades. In known systems of the internal control type, the point of steam extraction is separated from the down-stream turbine section by an intermediate wall which is sealed against the turbine housing and shaft. Steam is bled by way of several valves, placed on the housing exterior, into a duct which passes through the intermediate wall, thus reaching a point downstream of the point of extraction in a more or less throttled state. The rate of throttling by the valves thus determines the rate of steam flow into the extraction line. This design of an internal control requires a great amount of space in the axial direction due to the valves, the intermediate wall and the rerouting of the duct in the down-stream direction behind the intermediate wall.
In accordance with this great over-all axial length, the bearing span of the turbine shaft must also be great with the resultant danger of too low critical shaft speeds. The sealing of the intermediate wall against the turbine shaft also poses problems. Another disadvantage, occurring during operation, is the danger of housing deformations due to nonuniform heating resulting (for reasons based on manufacturing, design and proper operation) from the set of valves being arranged at the top of the housing, thus unbalancing its rotary symmetry. Finally, the steam flow through the set of valves and the duct is very undesirable from a flow-engineering standpoint.
It is an object of the invention to establish an internal control system for the extraction of steam from extraction turbines which will avoid the above-discussed disadvantages of the known arrangements.